Since the establishment of a method for synthesizing hydrophobic inorganic nanoparticles having a uniform particle size in an organic solvent containing a surfactant through a chemical process, extensive researches and studies are currently underway for practical applications. Among them, organic-inorganic composite materials, wherein a single inorganic nanoparticle exists at the center thereof and the surface of the nanoparticle is surrounded with polymers, have a wide range of application. This is because they can be effectively used as raw materials for fabrication of an organic-inorganic composite film and coating of substrate. They can also be used as raw materials for bioimaging. Thus, most of the researches have been focused on the practicalization of organic-inorganic composite materials.
Initially, a method of preparing hydrophobic or hydrophilic organic-inorganic composite materials by coating hydrophobic inorganic nanoparticles with polymers having a similar property to the nanoparticle surface or amphiphilic polymers via physical interaction was proposed. However, since the single organic-inorganic composite materials prepared by said method contain multiple inorganic nanoparticles inside, several problems occurred such as low quantum efficiency, size polydispersity and poor conversion yield. Since the adsorption of the polymer onto the nanoparticle is conducted by surrounding multiple inorganic nanoparticles with a single long polymer chain, such problems were regarded as being unavoidable. Thus, various methods were developed to overcome such problems, although they still suffered from poor conversion yield and inclusion of multiple inorganic nanoparticles inside a single composite. Accordingly, many attempts were made to develop a method of preparing an organic-inorganic composite material containing only a single inorganic nanoparticle therein in recent years.
The most regarded method is to prepare an organic-inorganic composite material containing a single inorganic nanoparticle therein by: conjugating several tens of organic ligands having a hydrophilic functional group and a thiol (SH) group to the surface of the nanoparticle through a metal-thiolate (M-S) bond; endowing hydrophilicity to the surface of the nanoparticle by converting the direction of the hydrophilic functional group of the organic ligand outward; and forming a covalent bond between the hydrophilic functional group of the nanoparticle and a biopolymer (see FIG. 2). Such covalent bond may be achieved through the formation of an amide bond or an ester bond between the hydrophilic functional groups of the nanoparticle and the biopolymer. For this, several organic ligands having one of the hydrophilic functional groups such as amine (NH2), carboxylic acid (COOH) or hydroxyl (OH) and a thiol group have been proposed. Such organic ligand can form a M-S bond with numerous nanoparticles such as a semiconductor nanoparticle (e.g., CdSe, ZnS or core/shell CdSe, CdS, CdSe/ZnS), a metal nanoparticle (e.g., Au, Ag) or a metal oxide nanoparticle (e.g., Fe2O3, Fe3O4) having metal-rich surface. Among them, since the carboxyl group exhibits high dispersability and solution stability, it has been widely used as a hydrophilic terminal group for conjugating a biopolymer to a nanoparticle by means of an amide bond.
However, said method has to undergo a step for activating the carboxyl group present on the surface of the nanoparticle in a weakly acidic aqueous solution, which results in aggregation and precipitation of many nanoparticles (W C Chan and S Nie, Science 281: 1998; and Jiang et al., Chem. Mater. 18: 872, 2006). Thus, precipitated nanoparticles cannot form an amide bond with other molecules due to their significantly reduced dispersibility. Further, another drawback is that a large quantity of nanoparticles is lost during the procedure of removing the precipitated nanoparticles and separating a supernatant containing a small amount of dispersed nanoparticles. Also, the hydroxyl terminal group has to form an ester bond in a strong acid solution or a strong base solution, which also causes the rapid aggregation and precipitation of nanoparticles.
In case that the hydrophilic terminal group is an amine group, it is possible to form an amide bond in a neutral aqueous solution, although it still suffers from the aggregation and precipitation of nanoparticles. In fact, when the aggregated nanoparticles are used as they are, the formation of an amide bond with a biopolymer is carried out only on the surface of the aggregate, which results in the occurrence of poor conversion yield and significantly low quantum efficiency.
To overcome such aggregation and precipitation problems, a method of preparing an organic-inorganic composite material having a M-S bond by reacting a hydrophobic inorganic nanoparticle with poly(ethylene glycol) (PEG) having a thiol terminal group has been suggested (see U.S. Pat. No. 7,041,371). However, since a long chain of hydrophilic thiol group has to penetrate into the surface of the nanoparticle surrounded with a surfactant even though the conversion yield is partially improved, numerous problems may still arise in that its conversion yield is still low and it does not fundamentally prevent the aggregation of nanoparticles.
Therefore, the present inventors have endeavored to overcome the prior art problems of aggregation and precipitation of hydrophilic nanoparticles having an amine terminal group on the surface thereof, and established a condition for dissociating hydrophilic nanoparticles dispersed in the form of an aggregate in an aqueous solution into a single nanoparticle and individually dispersing them. The organic-inorganic composite material prepared by covalently binding biopolymers to the nanoparticle individually dispersed according to said condition contains only a single inorganic nanoparticle without causing any aggregation or precipitation, thereby exhibiting improved properties such as excellent homogeneity, dispersibility, stability, biocompatibility and targetability.